Role of heat flow direction, monolayer film thickness, and periodicity in controlling thermal conductivity of a Si–Ge superlattice system

Abstract

Superlattices are considered one of the most promising material systems for nanotechnological applications in fields such as high figure of merit (ZT) thermoelectrics, microelectronics, and optoelectronics owing to the possibility that these materials could be tailored to obtain desired thermal properties. Factors that could be adjusted for tailoring the thermal conductivity of the superlattices include the monolayer film thickness, periodicity, heat flow direction, straining, and temperature of operation. In the presented study, nonequilibrium molecular dynamics (NEMD) simulations are performed to obtain an understanding of the effect of such factors on the thermal conductivity of Si–Ge superlattices at three different temperatures (400, 600, and 800 K). The NEMD simulations are performed using Tersoff bond-order potential. The thermal conductivity is found to increase with an increase in the number of periods as well as with the increase in the period thickness. The dependence of thermal conductivity on the direction of heat flow is found to be sensitive to the extent of acoustic mismatch at the interface (i.e., heat flowing from Si to Ge versus heat flowing from Ge to Si in a single period). Superlattices with Ge–Si interfaces (heat flows from Ge monolayer to Si monolayer in a period) are found to have lower thermal conductivity than superlattices with Si–Ge interfaces (heat flows from Si monolayer to Ge monolayer in a period). The superlattices thermal conduction, therefore, can be considered to have a characteristic somewhat similar to a thermal diode. Both compressive and tensile strains are observed to be an important factor in tailoring the thermal conductivity of the analyzed superlattices. Particularly, straining can help in reducing the thermal conductivity. The influence of straining is found to increase with increasing period thickness and periodicity.